1. Field of the Invention
This invention is related to network communication systems generally and, more specifically, to distribution network power line communication systems having bidirectionally addressable repeaters for providing bidirectional signal transmissions between a central station and remote devices.
2. Description of the Prior Art
Considerable effort has been devoted in recent years to develop power distribution network carrier communication systems due to the increased desirability of performing certain automated functions, such as the automatic reading of utility meters, selective load control, performing load continuity checks, and the like. However, using the power distribution network as a communication link presents several unique problems not encountered in transmission through paths exclusively intended for communication signals nor in existing high voltage transmission line communication systems. The distribution line, along with the large number of distribution transformers attached thereto, is designed for the transmission of sixty hertz electrical power. The distribution line presents poor and erratically varying high frequency impedance characteristics which rapidly attenuate the communication signals. Additionally, the introduction of electrical noise and signal interference substantially reduces reliability.
U.S. Pat. Nos. 3,967,264; 3,942,168; and 3,911,415, all assigned to the assignee of the present invention, disclose some form of communication system via the distribution network of an electrical utility. U.S. Pat. No. 3,911,415 discloses a power line communication system having signal reconditioning and frequency translating signal repeaters coupled to the conductors of a distribution network. Remote home terminals of different geographical zones are addressable at different frequencies to simplify address coding and isolation of potentially interfering carrier signals.
U.S. Pat. No. 3,967,264 describes a power line communication system which transmits carrier communication signals through addressable repeaters defining communication zones including remote terminals in corresponding geographical zones or subregions. The uniquely addressed repeaters modify segments of the transmitted signal which represent their own address as repeated transmissions are propagated between a central control terminal and uniquely addressed remote terminals. Time delays in the repeater retransmissions avoid interfering signal reception and assure the maintenance of a single repeater in operation during each repeating cycle. Timed waiting is provided by the repeaters for response from the remote terminals. The remote terminal logic is activated without requiring the repeater code. The repeater receivers are always on except during retransmission.
In the carrier communication system disclosed in U.S. Pat. No. 3,942,168, signal repeaters are disposed in signal communication with distribution network power line conductors at each distribution transformer site. In this system, the repeater, in addition to amplifying a signal, provides a bypass circuit for the interrogation and response signals around the associated distribution transformers which present a significant attenuation at the frequencies of the communication signals, especially in the direction from the primary to the secondary winding. The repeaters also include means for delaying a received signal, with the delay time interval selected such that the amplified signal is not applied to the power line at the same time the repeater is to receive another signal. Each repeater includes two channels, each comprised of a receiver, logic and transmitter sections for handling the bidirectional flow of interrogation and response signals between the central communication terminal and the remote communication terminals at the customer sites.
U.S. patent application Ser. No. 956,516, now U.S. Pat. No. 4,250,489, which is assigned to the same assignee as the present invention, discloses a power line carrier communication system having a branch or a pyramid configuration. Bidirectional addressable repeaters include an address recognition and a receive and transmit control logic circuits operated in a timed sequence of operations upon activation by an interrogation message transmitted from a central control terminal via one or more of the repeaters to a predetermined remote location. The branch organization of the repeaters includes groups of repeaters connected to the power line conductors of a distribution network so that each group defines a zone of repeaters. Each repeater zone is associated with remote terminals in geographical areas at progressively further distances from the central control terminal which defines the base or apex of the pyramiding branch configuration of the repeaters.
A first, or zone A, repeater group forms the primary branch paths each extending to a separate group of intermediate branch paths including as many groups of second, or zone B, repeaters as there are primary branch paths. Further branch paths are formed including third, or zone C, repeater groups similarly extending from each of the separate intermediate branch paths. The latter branch paths are the furthest terminal branch paths in one preferred embodiment. Remote communication terminals are coupled in signal communication with the power line conductors for receiving and transmitting signals between an adjacent repeater in any of the zones depending on the correspondingly similar geographic locations of the remote terminals and repeaters with respect to the power lines.
In this prior art communication system, an interrogation message originating at a central control terminal includes repeater address information having groups of repeater addresses in a predetermined segment of data bits in the message format. Each group is associated with a repeater zone and the binary coding of each group is associated with a separate repeater group. Thus, a reduced number of data bits is required and simplified address recognition logic is possible since large numbers of repeaters can have identical repeater addresses but are distinguishable because of their being in a different repeater address group and because of the different repeater address of the preceding repeater address group. False repeater activation is further prevented by each repeater's modification of its own repeater address segment of the retransmitted interrogation message to a predetermined null code such as all zero bits.
A major disadvantage of the prior art power distribution communication systems is that the electric utilities' power distribution systems must be reconfigured so as to fit the required geometry of the communication system. In other words, communication zones, geographical zones, or the like, must be defined so that the electric utilities' power distribution systems will resemble the pyramid-like structure or the matrix-like structure required by the prior art communication systems. This is often a time consuming and expensive task given the fact that power distribution systems have been expanding over several decades without any regard to the needs of a power distribution communication system.
Another major disadvantage of the prior art power distribution communication systems stems from the fact that the distribution network is a very dynamic system, both physically as well as electrically. The system is physically dynamic due to accidents caused by bad weather, vandalism, etc, additions to the system, and other types of construction. The system is electrically dynamic since the utility has the capability, through the closing and opening of switch sets, to completely re-route the flow of electrical power. Thus, a power distribution system which has been reconfigured so as to fit the required geometry of a prior art communication system may require often and extensive revision so as to maintain the required geometry due to the dynamic nature of the system.
The present invention is for a distribution network communication system which requires no specific configuration as required by prior art systems. This represents a considerable savings of time and man-power to the user of the communication system since the communication system is configurable to the existing power distribution network rather than vice versa. This flexibility represents a major advantage of the present invention over the prior art systems.